Since it's always three taps spaced equally I'm going to guess it's something mechanical, like something with the motor in their refrigerator.

The mechanical suggestion makes the most sense. i think my fridge makes a kind of tapping like that when it is powering down, or whatever the name for that is.

But knowing the frequency would help.

Is it really disturbing enough to you that you have to ask about it? Could you just keep your radio on so you don't notice it?

It's just really hard to tell from your original description. If it's just 3 small taps a few times a day, I would think you could just let it go.

It's disturbing when it's quiet at night, I'm laying in bed, and trying to fall asleep. But there is just no pattern to it. I heard it yesterday morning when I was getting ready for work. Didn't hear it last night or this morning. It's three taps that go on for -- I don't know -- maybe an hour or so at a time, then it stops (almost like a machine got turned off). Never really timed it. Always 3 taps, pretty regular in how they are spaced out. It's a curiousity thing more than anything else...but you know how when you are trying NOT to pay attention to something, that's all you key in on??? That's what is happening here.

Get a white noise machine/radio/tapes and just forget about it... it's probably a powering-down of some mechanical thingy in the walls at a sticky place.

Wild guess - bread machine? When it's going through the dough kneading cycle? Those can be set on a timer, I think (it's been years since I used mine) so could run at goofy hours depending on when they want they're bread done. ;D